The present invention is directed to an edge interference filter for an optical communication transmission in wavelength-multiplex which filter comprises a layer sequence arranged between adjacent outer media or substrates with the sequence being composed alternately of layers of high index of refraction and layers of low index of refraction with the two outer layers of the layer sequence which are adjacent to the outer media being of the higher index of refraction.
Filters having a layer sequence of alternating high and lower index of refraction are known. Examples are disclosed in an article by F. H. Mahlein, "A High Performance Edge Filter for Wavelength-Division Multi-Demultiplexer Units", Optics and Laser Technology, February 1981, pages 13-20. Under certain conditions, pairs of spectrally complementary edge interference filters are required for optical communication transmissions and wavelength-division multiplex. Such pairs in an unglued execution are known and examples are disclosed in a book by H. A. Macleod, Thin Film Optical Filters, Hilger, London, 1969, pages 111-153.
The theory of glued edge interference filters (see the above mentioned article by Mahlein) shows that filters for transmitting short-wave can be realized with a standard layer materials composed of SiO.sub.2 and TiO.sub.2 only for narrow interchannel spacings of about 40 nm given a wavelength of 1300 nm and a 20.degree. angle of incidence. Filter for transmitting the long-wave can only be realized for greater interchannel spacings of about 120 nm given a wavelength of 1300 nm and a 20.degree. angle of incidence.
By contrast, edge interference filters transmitting long-wave for narrow interchannel spacings and filters transmitting short-wave for large interchannel spacings cannot be realized with the layer structures hitherto described.